meanfp calcmode class is missing

[imaliyov]

The corresponding class for the Perturbo meanfp calcmode is missing, the link in the tutorial for meanfp point to the general perturbopy tutorial page, but tutorial is not there either.

https://perturbo-code.github.io/mydoc_scattering.html

This is one of the standard modes of Perturbo so it would be nice to have:

1. A corresponding perturbopy class
2. Perturbopy tutorial
3. Corresponding perturbopy test

[hurricane642]

Hey Ivan,
Thank you a lot!
I'll work on this soon!